Electric cable



Dec. 27, 1938. w. c. HAYMAN ELECTRIC CABLE Filed March 19, 1935 11m/ent or: William C.Ha1j-mah yg/ 5.9M b9 maize/Attorney.

Patented Dec. 27, 1938 William C. Hayman, Schenectady, N. Y., assigncil to General Electric Company, a corporation New York Application March `19, 1935, scn'ai Nc. 11,763

2y claims. (C1. 173-264) The present invention relates tov electric cables and in particular to cables which are adapted for use in secondary underground network systems operating at low voltages, as 110 volts and 5 220 volts for example, and heavy currents.

In such systems it is the established practice in order to avoid service interruption to provide electric cables which are so constructed that if a short-circuit or other fault occurs, a resulting arcing will be allowed to persist until the fault has been cleared by the burning away of the cable structure at the fault. The severed ends which are still connected to the network at their remote ends remain in service. Ii the cable in-` ,v sulation consists wholly or preponderantly of combustible organic material, as for example, paper, cotton, or the like, then combustible gases are evolved by the arcing and the resulting heats ing of the adjacent cable structure.

Dangerous $0 explosions have resulted when suilicient amounts I of such gases have become mixed with air an become ignited in a confined space.

In accordance with clark United states Patent 1,931,373, of October 17, 1933, evolution of explosive gases in an electric cable is avoided by using in such cable a liquid halogenated cyclic. compound which has a suiciently highy chlorine content to' yield only substantially non-inflammable gases under the conditions above described. v

paper or other organic insulating material pro-l viding the amount of organic material of this` type is not suiiiciently high to render the propor- Such material may be used in conjunctiony with tion of combustible gas in the gaseousmixture high enough to permit explosions or res to occur. Although it is practicable in the fabrication of an electric cable to ll the entire space between the conductor and the surrounding sheath with paper which is impregnated with a liquid halo/- genated compound of this class, or to provide ducts iilled with such compound, such construetions are not applicable for every class of service@ It has been suggested to provide in thespace surrounding the conductor an inorganic insucompound.' such a material does not have as high ay dielectric strength as is desirable for electrical distri- U To provide' suicient insulation 1 bution service. value would entail an increase in the thickness of the insulation layer which would unduly increase the diameter and cost of the cable.

In accordance with my present invention I have provided a composite insulation consisting lating material of' high porosity, such as asbestos, which may be impregnated with such halogenated However, it has been found that I and Fig. 2 fis a cross-section preponderantly of tire-proof inorganic insulation and a relatively thin layer of organic, plastic insulation of high dielectric strength, thereby greatly increasing the dielectric value of-l the insulation as a Whole without materially adding to 5 its'cost, and without sacricing the fire-proof quality of the insulation.

For the layer of organic. insulation I .may employ cotton fabric'impregnated with flexible, synthetic lation/1 may employf'asbestos or'similar porous v inorganic brous material.` Such inorganic material isfimpregnated with chlorinated diphenyl, orsimila'r non-combustible, inert liquid.

-In the accompanying drawing, Fig.` l is a side 16 View partly in section of a length of electric cable v of such cable em-r bodying my invention.

' Referring to the ldrawin'g the cable conductor or core I, ordinarily consists of a bundle of cop- 20 per wires.` Surrounding the conductor l is a layer 2 of an organic plastic insulation of high /dielectric strength, such, for example as a natural A. or synthetic resin and preferably an alkyd resin. Other .suitable materials may /be used as for 25 example,` cellulose acetate, or #acrylic acid condensation products. A wrapping-of cotton tape impregnated with an alkyd resin made by the chemical reaction of phthalic anhydride, glycer, ine',y succinic.aloid,1ethyleneglycol, abietic acid, 30 an China-wood oil, may be used. Such a resin is described in U.. S. Patent 1,975,569, patented October 2, 1934. --ThislayerZ which may be lapwound, :ss-indicated, is about 1&4 inch in thickness. The describedmaterials are .unaffected by the 35 solvent properties of chlorinated diphenyl compounds. 1

Superimposedonithe layer 2 are two windings 3, 4 of' asbestos yarn, or thread, each of which is about the same thickness. Instead of asbestos 40 @other formsl of inorganic insulation may be used, as for examplegspun glass, or mineral wool. Inystead of being yapplied-"in thread-like form, th

- mineral layer may be lapplied as a tape, felt, r

otherl physical form. An: external/ protective 45 sheath` made for example of metal, surrounds the'who'le. v The-mineraliibre layers are impregnated with a liquid yhalogenated cyclic hydrocarbon, as for example, chlorinated diphenyl, `chlorinated di- 50 phenyl oxide, chlorinated diphenyl methane, chlorinated diphenyl ketonetrichlor benzene, or various liquid mixtures of suchY compounds. Preferably I employ liquid chlorinated diphenyl containing about 50 to 60 per cent chlorine. 55

resin, and for the layer of inorganic insul0y I' Suitable impregnating compounds of this type are described in the above Clark Patent 1,931,373.

The advantage accruing from the cable insulation structure of my present invention will be apparent from a consideration of the fact that the dielectric strength of the combination of the two unlike insulation layers is about ve kilovolts while the dielectric strength of the impregnated asbestos alone (even if of the same total thickness as the total thickness of the asbestos and resin) is only about 1.5 kilovolts.

While in the preferred form of my invention the two types of cable insulation have been shown in combination in a superimposed relation in which the mineral layer is wholly external to the resin-impregnated fabric, I desire by the appended claims to cover also other combinations of such layers. For example, in some cases the resin-impregnated fabric may be sandwiched between two layers of mineral insulation.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. An electric cable which is adapted for use in secondary underground network systems operating at low voltages and heavy currents, said cable consisting of a conductive core, a wrapping about 1,64 inch thick on said core, said wrapping consisting preponderantly of cellulosic material impregnated with a resin resulting from the reaction of phthalic anhydride, glycerine, succinic acid, ethylene glycol, abietic acid and China-wood oil, a second wrapping of asbestos impregnated with liquid chlorinated diphenyl containing about 50 to 60 per cent chlorine, said combined insulating wrappings having a dielectric strength approximately three times the dielectric strength of the impregnated asbestos alone when the latter is of the same thickness as the total thickness of the said combined wrappings, and an external metallic sheath surrounding the whole.

2. An electric cable which is adapted for use in secondary underground network systems operating at low voltages and heavy currents, said cable consisting of a conductor, a relatively thin mass of organic insulation superposed directly on the conductor, said insulation consisting of cotton fabric impregnated with a flexible alkyd resin, an insulating mass consisting substantially of asbestos impregnated with liquid chlorinated diphenyl compound superposed directly on said organic insulation, said combined insulating masses having a dielectric strength at least three times the dielectric strength of the impregnated asbestos alone when the latter is of the same thickness as the total thickness of the said combined masses, and an external metal sheath surrounding the whole.

WILLIAM C. HAYMAN. 

